1. Field of the Invention
The present invention relates to an optical element, a projection optical system, an exposure apparatus, and a device fabrication method.
2. Description of the Related Art
A projection exposure apparatus has conventionally been employed to fabricate a micropatterned semiconductor device such as a semiconductor memory or logic circuit by using photolithography. The projection exposure apparatus projects and transfers a circuit pattern formed on a reticle (mask) onto a substrate such as a wafer by a projection optical system.
A minimum feature size (resolution) that the projection exposure apparatus can transfer is proportional to the wavelength of light for use in exposure (exposure light), and is inversely proportional to the numerical aperture (NA) of the projection optical system. According to this principle, the shorter the wavelength and the higher the NA, the better the resolution. To keep up with demands for advances in micropatterning of semiconductor devices, the wavelength of the exposure light has shortened, and the NA of the projection optical system has increased. In recent years, a KrF excimer laser (wavelength: about 248 nm) or an ArF excimer laser (wavelength: about 193 nm), for example, is used as the exposure light.
The projection optical system is required to minimize birefringences. The birefringences of the projection optical system are roughly classified into the glass birefringence in an optical element (transmitting element), and the phase difference between two polarization components in an optical thin film (for example, an antireflection film) formed on the surface of an optical element.
The glass birefringence as the first birefringence will be explained first. Synthetic silica and fluoride-based crystal materials are mainly used for transmitting elements of the projection optical system, which are used for light having a wavelength of 250 nm or less. The transmitting element is generally known to have an intrinsic birefringence attributed to its crystal orientation and a stress birefringence attributed to its internal stress. According to a report involved, fluorite (calcium fluoride) as one fluoride-based crystal material, for example, has an intrinsic birefringence in an amount that is non-negligible from the viewpoint of the optical performances. Amorphous materials such as synthetic silica that are widely used for light in the ultraviolet range have practically no intrinsic birefringences attributed to their crystal orientations. However, a stress birefringence thought to be attributed to a thermal stress has been experimentally observed in synthetic silica, and the influence of the amount of birefringence exerted on the optical performances of the projection optical system is not small.
The phase difference between two polarization components in an optical thin film as the second birefringence will be explained next. In general, when a light beam propagates through an antireflection film formed on the surface of a transmitting element used in the projection optical system, a difference (a phase difference between two polarization components) occurs between the transmission phases of the S- and P-polarization components of the light beam. The phase difference between two polarization components as in this case can be processed in the same way as in a birefringence generated in the transmitting element, and therefore can be regarded as a birefringence.
As the NA of the projection optical system increases, the incident angle of a light beam which enters an optical element of the projection optical system increases. Along with this trend, a projection optical system having an NA higher than 0.85 is provided with an optical element which receives, at an incident angle of 55° or more, a light beam that propagates through its outer periphery. It is very difficult to decrease the phase differences between two polarization components in an optical thin film at all incident angles on the surface of an optical element which receives a light beam at such a large incident angle. If, for example, an antireflection film is formed on the surface of synthetic silica, fluorides and oxides are known as materials (optical thin film materials) which can be used for vacuum ultraviolet light having a wavelength of 193 nm. Note that, if an antireflection film is uniformly formed on the surface of synthetic silica using these optical thin film materials, it is practically impossible to decrease the phase differences between two polarization components at all incident angles on a surface which receives a light beam at a maximum incident angle of 55° or more.
Under the circumstances, Japanese Patent Laid-Open No. 2004-157349 proposes a technique of reducing the overall birefringence of the projection optical system by canceling the glass birefringence in each optical element and the phase difference between two polarization components in an optical thin film on each surface (i.e., canceling the glass birefringence and the phase difference between two polarization components). Japanese Patent Laid-Open No. 2004-157349 discloses a method of controlling the intrinsic birefringence and stress birefringence of a crystalline glass material using the phase difference between two polarization components in an optical thin film (antireflection film) formed on the surface of an optical element. Japanese Patent Laid-Open No. 2004-102016, for example, also proposes a technique of improving the antireflection characteristic (i.e., decreasing the reflectance) of an antireflection film serving as an optical thin film, although this technique does not decrease the phase difference between two polarization components in an optical thin film.
Unfortunately, the birefringence components which can be canceled by the technique disclosed in Japanese Patent Laid-Open No. 2004-157349 are mainly limited to low-order birefringence components. This is because the configuration of an antireflection film can control low-order components of the phase difference between two polarization components, but cannot control high-order components of the phase difference between two polarization components as long as a good antireflection characteristic is maintained.
FIG. 11 is a graph showing the phase differences of three types of antireflection films AR1, AR2, and AR3 used for light having a wavelength of 193 nm as a function of the light beam incident angle (i.e., the dependences of the phase differences on the incident angle). In FIG. 11, the ordinate indicates the phase difference [mλ] and the abscissa indicates the incident angle [°]. The antireflection films AR1, AR2, and AR3 are formed to have different phase difference characteristics while suppressing their reflectances (while maintaining their antireflection characteristics) with respect to light beams at incident angles up to 55°.
Referring to FIG. 11, obviously, the phase differences of the antireflection films AR1, AR2, and AR3 cannot practically be set to zero in the incident angle range of 0° (inclusive) to 55° (inclusive). It is possible to reduce the overall birefringence of the projection optical system by appropriately selecting the antireflection films AR1, AR2, and AR3 as antireflection films to be formed on a plurality of optical elements of the projection optical system. Note that, when the light beam incident angle exceeds 45°, the phase differences of the antireflection films AR1, AR2, and AR3 decrease as the light beam incident angle increases, as shown in FIG. 11. When the antireflection films AR1, AR2, and AR3 are appropriately selected to decrease the overall phase difference between two polarization components in the projection optical system, the phase difference components at incident angles larger than 45° remain as those hard to cancel. Such phase difference components hard to cancel appear as high-order components of the exit pupil plane distribution of the phase difference between two polarization components, and significantly degrades the optical performances of a projection optical system including an optical element which receives a light beam especially at a maximum incident angle of 55° or more.
Japanese Patent Laid-Open No. 2004-102016 also discloses a technique of decreasing the reflectance of an optical element which receives a light beam at a large incident angle in a low-NA optical system by forming, on the optical element, an antireflection film having a film thickness distribution in which the thickness increases toward the periphery of the optical element. Note that the technique disclosed in Japanese Patent Laid-Open No. 2004-102016 can decrease the reflectance of an optical element with respect to a light beam which enters it at a large incident angle, but cannot decrease the phase difference between two polarization components in an optical thin film formed on it. More specifically, Japanese Patent Laid-Open No. 2004-102016 sets the thickness of an antireflection film in the periphery of an optical element to about 1.05 times that of the antireflection film at the center of the optical element. However, such a film thickness distribution cannot sufficiently decrease the phase difference between two polarization components of a light beam at an incident angle of 55° or more. This makes it impossible to sufficiently decrease the overall phase difference between two polarization components in an optical system including an optical element which receives a light beam at an incident angle of 55° or more.